This invention relates to rotary sheet-fed offset printing presses, and more particularly to a new and improved anti-marking vacuum transfer apparatus for supporting freshly printed sheets as they are moved between processing stations within the press.
During the movement of sheets being printed through a rotary sheet-fed offset printing press, it is conventional to employ a transfer or delivery system which engages and supports the wet ink side of the sheet as the sheet is moved between processing stations. Typically, a "transfer" system denotes an apparatus disposed between the several printing stations in the press and which functions to receive a freshly printed sheet from one impression cylinder and move the sheet to the next printing station for additional printing by a further impression cylinder. A "delivery" system typically denotes an apparatus which receives the freshly printed sheet from the last impression cylinder of the press, and delivers the sheet to the press delivery station, typically a sheet stacker. As used hereinafter, the term "transfer" is intended to include both apparatus used to transfer a sheet between printing stations of the press and an apparatus used for delivering the sheets to the press delivery stacking station.
One problem inherent in all transfer systems which engage and support the printed side of the sheet is that of marking and smearing the freshly applied ink. In the past, efforts to reduce sheet marking and marring have included employing apparatus such as those referred to in the trade as skeleton wheels and cylinders, and which have sheet engaging surfaces intended to minimize the area of sheet contact while still providing sheet support. Exemplary of such prior art devices are those discussed in the Background of the Invention of U.S. Pat. No. 3,791,644 issued Feb. 12, 1974 to Howard W. DeMoore entitled "SHEET HANDLING APPARATUS".
Another approach employs a transfer system having a cylinder with a specially prepared friction reducing support surface covered by a fabric cloth, known in the trade as a "net", and which is more fully described in U.S. Pat. No. 4,402,267 issued Sep. 6, 1983 to Howard W. DeMoore, entitled "METHOD AND APPARATUS FOR HANDLING PRINTED SHEET MATERIALS". That system, which is marketed under license by Printing Research, Inc. of Dallas, Tex. under its registered trademark "SUPER BLUE", actually maximizes the area of contact between the wet ink side of the sheet and the net covered surface of the transfer cylinder.
While the "SUPER BLUE" system has received wide spread industry acceptance and has enjoyed substantial commercial success, after prolonged use it is often necessary that the fabric net be replaced due to a build-up of ink on the net surface, or as a result of the net becoming excessively worn and/or torn. While the "SUPER BLUE" system allows the fabric net to be replaced relatively quickly, replacement of the net still requires that the press be shut down, thereby resulting in periodic press down time.
In many printing applications, only one side of the sheet receives ink from the blanket cylinders during each pass through the printing press. Applicants have found that in those situations where only one side of the sheet is to be printed, use of a transfer system which engages and supports the printed side of the sheet may be unnecessary and a transfer system can be used which engages and supports the nonprinted side of the sheet. For example, in non-perfector type printing presses, only one side of the sheet is printed during each pass through the press. In such presses, conventional transfer systems which support and engage the printed side of the sheet can be eliminated, and a transfer system which engages and supports only the nonprinted side of the sheet can be used.
In U.S. Pat. No. 2,933,039 issued Apr. 19, 1960 to Clayborn et al., entitled "SHEET TRANSFERRING MECHANISM", there is disclosed a transfer system for preventing sheet marking and which is intended to be a substitute for conventional transfer apparatus which engage and support the printed side of the sheet. That patent discloses a stationary curved sheet guide having a solid surface mounted adjacent to the path of the sheet transfer grippers and which supports the nonprinted side of a freshly printed sheet as it is pulled by the grippers from the impression cylinder. As discussed in that patent, provision is made for creating a negative pressure between the sheet and the solid surface of the sheet guide so that the sheet is drawn into engagement with the sheet guide as it is pulled by the grippers from the impression cylinder. Since only the nonprinted side of the sheet is engaged and supported by the sheet guide, marking and marring of the freshly printed surface cannot occur.
In U.S. Pat. No. 4,572,071 issued Feb. 25, 1986 to Cappel et al., entitled "DEVICE FOR GUIDING SHEET PRINTED ON ONE OR BOTH SIDES", there is disclosed an improvement over the foregoing Clayborn et al. patent, and which suggests employing a stationary curved sheet guide having an apertured solid support surface through which air can be drawn to create a negative pressure on the sheet, thereby to draw the nonprinted side of the sheet against the sheet guide. In this respect, this patent suggests that the sheet guide be formed as the surface of a plenum chamber coupled to a plurality of fans which can be selectively operated to either provide a negative pressure within the plenum chamber, or a positive pressure within the chamber such that the sheet can, respectively, be either drawn against the surface of the sheet guide in the case of single sided printing, or "floated" above the surface of the sheet guide in the case of two sided printing.
Applicants have found that with stationary sheet guide apparatus of the type disclosed in the Clayborn et al. and Cappel et al. patents, since the sheet is drawn onto and pulled against the substantially solid support surface of the sheet guide, the nonprinted side of the sheet may tend to be scratched and marred as it slides over the solid support surface. Further, use of stationary sheet guide apparatus of the types suggested by the Clayborn et al. and Cappel et al. patents may result in the sheet being pulled partially or fully from the transfer grippers due to the high frictional force created between the sheet and the supporting surface of the sheet guide, thereby resulting in sheet misalignment and destroyed registration for subsequent printing by the next printing unit.
As will become more apparent hereinafter, the present invention provides a new and improved transfer apparatus for supporting the nonprinted side of a sheet which solves the foregoing problems in a novel and unobvious manner.